Optical communication links generally utilized lasers with small beam divergences in a line-of-sight mode. For all wavelength regions, except the solar blind UV, the receiver must be physically located within the direct beam cone. This implies that very accurate tracking and pointing mechanisms must be employed so that the and transmitter are locked onto each other. This is particularly true for communication between rolling and pitching ships, manoeuvering aircraft, helicopters and trucks in rough terrain.
Maximum ranges are obtained when the laser beam has a minimum value of divergence, for example, in the range of 0.1 milliradian and an appropriate receiver is provided which senses this direct beam. An elaborate pointing mechanism must be provided at the transmitter to aim the narrow pencil beam. Delicate and complicated supporting equipments may be required which might be prohibitively expensive.
Two of the most important atmospheric parameters that determine range are the ozone concentration and the scattering co-efficient. Ozone completely eliminates a photon by absorption. Scattering changes the direction of a photon with the possibility of this photon still being used as a signal if it strikes the detector. In the solar-blind region of the UV, the scattered radiation can be so significant that the communication can be effected with the receiver being outside of the direct cone. Signal photons are available in a much larger (scattering) cone. This permits a relaxation of the demands for accurate pointing (boresighting of the transmitter-receiver).
FIG. 1 shows, symbolically, a transmitter with some value of beam divergence. The solid angle that is subtended by this divergence constitutes the direct beam. In this cone, a photon reaches the detector without being scattered. All non-UV communication links operate in this mode. Because photon scattering increases inversely as the fourth powe of the wavelength, UV scattering and UV communication links can operate in the scattered mode as well as the direct mode.
There are two types of scattering in this spectral region: (a.) Rayleigh scattering of atmospheric molecules and (b.) Mie scattering of atmospheric particulates which is strongly biased in the forward direction. The Rayleigh scattering is most important for large angular scattering as in the UV communication links around and over buildings. Such a UV communication system has been designed that provides for non- line-of-sight omni-directional communications via UV radiation in the solar-blind region of the electromagnetic spectrum and o 10 is disclosed in U.S. Pat. No. 4,493,114. The 253.7 nm UV radiation provides reliable short range communications with the disclosed UV source and the UV is totally absorbed beyond relatively short distances.
Mie scattering is an additional scattering for UV communication links which are almost line-of-sight and an important feature of this inventive concept for a source of signal photons is provided in a large cone surrounding the direct beam. With this feature the requirement that the detector and transmitter be accurately boresighted to collect the signal photons, is drastically relaxed. This feature also means that communications between moving, rolling and pitching transmitters and receivers are readily effected without signal fadeout with an increase in operable range.
At angles greater than beam divergence the path loss would increase very rapidly. FIG. 2 shows the path loss using a transmitter with a divergence of 2 degrees and is a demonstration of a typical result from the analysis of a typical mid-range UV communication link., which is: (1) Path loss of a communication link is calculated assuming that each UV photon either traverses the atmosphere unattenuated or is absorbed by an ozone molecule, or is singly scattered by Rayleigh (molecular) or Mie (particulate). (2) The ozone concentration is nominally 10 PPB, a typical value in mid-ocean. (3) The scattering coefficient is 0.434 inverse km. (4) The date rate is 2400 baud, which can support voice. (5) Maximum permissible path loss to obtain reasonable bit error rates for recognizable voice is 160 dB.
Detection of UV photons beyond the communication distance (a distance beyond the 160 db loss distance) depends largely on the detector integration time, the time required to perform an azimuthal scan, and the time the transmitter is operating. Detection range is defined as the distance where path loss is 20 db greater than the communication limit, in other words at 180 db.
The path loss for a link distance of 10 km. is shown for the angular misalignment between transmitter and receiver. The transmitter beam divergence is 2 degrees. The curve shows that a 20 degree transmitter-receiver misalignment can be tolerated up to a maximum 160 dB loss. If scattering were absent, the path loss would continue to increase rapidly as indicated by the dotted line (the direct, non-scattered beam). The scattered photons provide usable energy in a much larger cone which increases the link distance.
The curve of FIG. 2 shows that a 20 degree transmitter-receiver misalignment can be tolerated at the maximum 160 dB loss. Analysis shows that the same results are obtained when the transmitter beam divergence is increased to a maximum value of the permissible pointing error. It is this virtue that permits the usage of inexpensive, focussed non-laser , incoherent sources.
FIG. 3 depicts the maximum transmitter-receiver misalignment permissible as a function of link distance for mid-ocean. For example, for a link distance of 12 km, the receiver may be aimed 10 degrees from the line-of-sight and the communication link remains effective. On the other hand, as indicated above, the transmitter beam divergence could be increased to a maximum value of 10 degrees.
In other words, the data presented in FIG. 3 represents the maximum transmitter misalignment permissible as a function of link distance for mid-ocean. For example, for a link distance of 10 km the transmitter can be aimed 20.degree. from the line-of-sight and the communication link would still be effective, or stated differently,the same results can be obtained with a transmitter of plus or minus .degree. beam divergence.
Thus, a continuing need exists in the state-of-the-art for a UV transmitter having a directional capability of about 20.degree. to assure more reliable mid-range UV communications.